Shell and tube friction stir welded (FSW) heat exchangers have been developed for marine grade applications such as ocean thermal energy conversion, thermal desalination and other relatively low temperature processes. In addition, the FSW process is being used in other heat exchangers that operate under higher temperatures and pressures. In the FSW process as applied to shell and tube heat exchangers, a solid state welding or stirring process is used wherein the ends of tube walls are “stirred” into surrounding tube sheet material without introducing dissimilar metals and without adverse effect to metal grain structure.
In the heat exchanger, the tubes are inserted into tube sheets on either end of the bundle of tubes in a manner similar to traditional shell and tube heat exchanger designs. The ends of the tubes are normally flared prior to the FSW process. This flaring or tube-end expansion allows the tubes to stay in place while reacting the forces applied during the FSW process.
The FSW process eliminates crevices that would ordinarily exist between mechanically rolled tube ends and the surrounding tube sheet material. Elimination of crevices is desirable to obtain a heat exchanger that can have long-life in a corrosive seawater environment. However, the process of flaring or expanding the ends of the tubes can sometimes introduce an undesirable crevice or locally deformed zone that can become a site for preferential crevice corrosion, particularly in a seawater environment.